What version of Elasticsearch did you install? What version of Gitlab are you using?

In recent versions of Elasticsearch the parsing has been made more strict to prevent unexpected errors so the API requires script true and false values and rejects lenient boolean values like 0, 1, "on", "off".

It looks like your version of Gitlab is using an Elasticsearch 5.0.3 client so I suspect you are using a 6.x version of Elasticsearch and that is why you are getting the error. Note that on the guide you linked it says

Elasticsearch 6.0+ is not supported currently. We will support 6.0+ in the future.

Also note (from the same link) the Elasticsearch version requirements for different Gitlab versions:

Why is it that one always will miss the most vital information? Yes, version incompatibility will most likely be the root problem here (I installed Elasticsearch from the repository and it's the newest version, 6.1.1). I'll try it with a version <6 and will report back if I have been successful or not.

Thanks!

EDIT: I now installed Elasticsearch 5.5 and it installed correctly, thanks again!